Chain drives, in particular those based on roller chains, are used for the purpose of driving and conveying in industrial applications, wherein a plurality of chain strands being used in various fields of application. A chain drive unit normally comprises an endlessly circulating chain deflected around a plurality of spaced apart chain wheels and a plurality of driving or conveying elements actuated by means of the chain or connected to the chain. Drive chains are, in operation, subject to wear through the relative movement of the individual components in the chain hinge and to other various chain elongation factors, such as initial elongation, stretching, bearing slackness and bearing abrasion. These wear and elongation factors result in an elongation of the chain and, in the end, lead to a failure of the drive unit. Wear of a chain also depends on the drive unit in which the chain is used, on the loads acting on the chain and on the surroundings in which it is operated. Hence, chain wear and, consequently, the failure of the drive unit cannot be predicted with certainty.
Within the framework of modern factory automation, machines and systems are increasingly configured as fully automatic means and, consequently, complex chain drives are used to an increasing extent. The high investment cost of such fully automated machines and systems necessitates the constant use of the latter with the least possible number of inadvertent downtimes. Such inadvertent downtimes lead not only to direct financial losses but also to indirect problems, such as the interruption of the logistics chain and even an incapability of observing delivery times, and thus to further financial losses. However, the use of wear-prone chain drives cannot be dispensed with in the foreseeable future, since there is no alternative that could be used for driving such industrial systems and for conveying products.
In addition, even minor wear of the drive chains in automated machines and industrial plants necessitates a manual readjustment of sequences of operations that are synchronized with the chain position.
Since it is neither possible to dispense with the use of drive chains nor to avoid or precisely determine the wear and the chain elongation of the drive chain, the condition of the drive chain must be regularly monitored so as to allow a systematic readjustment of the synchronized sequences of operations and so as to be able to plan and execute inspections and replacement of a worn drive chain in good time.
A suitable method of monitoring the elongation of a circulating drive chain is known from U.S. Pat. No. 5,291,131. Here, the drive chain has provided thereon two marks, which are spaced apart in the longitudinal direction of the chain and the position of which is detected, during operation, by two inductive or optical sensors which are also arranged in spaced relationship with one another. The speed of circulation of the chain as well as the chain elongation can be determined from the measurement values of the two sensors via a data acquisition means connected thereto. Reference EP 1 464 919 A1 describes a similar method of monitoring the wear of a drive chain. To this end, opposed sides of the chain have provided thereon two markers of magnetic material, which, when passing two inductive sensors, cause an electric signal to be generated. The sensors are arranged in spaced relationship with one another on opposed sides of the drive chain such that, initially, the sensors are triggered simultaneously. As soon as there is a time delay between the triggering of the sensors due to wear elongation of the chain, said wear elongation of the chain can be determined through a positional displacement of the sensors. Another device for measuring the wear elongation of a drive chain by means of two optical sensors is described in U.S. Pat. No. 7,540,374 B2. Here, the first sensor detects the first chain hinge of a chain link and a second sensor determines the position and the distance of the second chain hinge. In addition, also the distance of a plurality of chain links at two spaced-apart measurement positions can be determined.
It is also known to determine the wear of a drive chain via the measurement of the force, the path or the angle of rotation of chain tensioners or of two rotary position sensors on the driving wheel and on the load wheel. However, a precondition for this is that, on the one hand, a chain tensioner is actually required or rotary position sensors can actually be used and, on the other hand, that these elements are then influenced by wear and chain elongation, respectively. In addition, the measurement depends on the total length of the chain in both cases, since in the case of a longer chain the absolute values of the path and of force progression are greater. Hence, such methods must be calculated and adjusted with respect to the respective case of use and with respect to the effect of the measurement results and, consequently, these methods cannot be applied generically. In addition, both said methods measure not only the direct wear-dependent chain elongation but also the wear of the chain wheels.
Depending on the respective sensors and measurement principle used, these devices and methods known from the prior art entail a plurality of different drawbacks. Conventional measurement systems with fixed distances between the sensors necessitate, for a precise measurement of chain elongation, a drive unit with a constant speed, and they react with measurement errors to irregularities in the drive system, e.g. a relative slip between the driving wheel and the drive chain. In addition, optical sensors are, in many cases of application, not suitable for practical use in driving and conveying systems, since the industrial surrounding conditions contribute, due to dust and dirt, to a possible failure or incorrect measurements of the optical sensors. Inductive sensors exhibit, in addition to a switching sensitivity in the direction of measurement, also an inherent switching sensitivity perpendicular thereto, so that inductive sensors are not only sensitive to vibration but they also tend to execute incorrect measurements.